From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934820AbeEWUOs (ORCPT ); Wed, 23 May 2018 16:14:48 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:44582 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934424AbeEWUOn (ORCPT ); Wed, 23 May 2018 16:14:43 -0400 Date: Wed, 23 May 2018 13:04:58 -0700 From: "Paul E. McKenney" To: Steven Rostedt Cc: Joel Fernandes , linux-kernel@vger.kernel.org, "Joel Fernandes (Google)" , Peter Zilstra , Ingo Molnar , Boqun Feng , byungchul.park@lge.com, kernel-team@android.com, Josh Triplett , Lai Jiangshan , Mathieu Desnoyers Subject: Re: [PATCH 1/4] rcu: Speed up calling of RCU tasks callbacks Reply-To: paulmck@linux.vnet.ibm.com References: <20180523063815.198302-1-joel@joelfernandes.org> <20180523063815.198302-2-joel@joelfernandes.org> <20180523155734.GK3803@linux.vnet.ibm.com> <20180523124531.7b0e972a@gandalf.local.home> <20180523170303.GR3803@linux.vnet.ibm.com> <20180523151337.469bba34@gandalf.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180523151337.469bba34@gandalf.local.home> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18052320-0016-0000-0000-000008C9EF08 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009073; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000261; SDB=6.01036637; UDB=6.00530330; IPR=6.00815764; MB=3.00021263; MTD=3.00000008; XFM=3.00000015; UTC=2018-05-23 20:14:39 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18052320-0017-0000-0000-00003EE04ED3 Message-Id: <20180523200458.GD3803@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-05-23_07:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 impostorscore=0 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1709140000 definitions=main-1805230198 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, May 23, 2018 at 03:13:37PM -0400, Steven Rostedt wrote: > On Wed, 23 May 2018 10:03:03 -0700 > "Paul E. McKenney" wrote: > > > > > > diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c > > > > > index 5783bdf86e5a..a28698e44b08 100644 > > > > > --- a/kernel/rcu/update.c > > > > > +++ b/kernel/rcu/update.c > > > > > @@ -743,6 +743,12 @@ static int __noreturn rcu_tasks_kthread(void *arg) > > > > > */ > > > > > synchronize_srcu(&tasks_rcu_exit_srcu); > > > > > > > > > > + /* > > > > > + * Wait a little bit incase held tasks are released > > > > > > > > in case > > > > > > > > > + * during their next timer ticks. > > > > > + */ > > > > > + schedule_timeout_interruptible(HZ/10); > > > > > + > > > > > /* > > > > > * Each pass through the following loop scans the list > > > > > * of holdout tasks, removing any that are no longer > > > > > @@ -755,7 +761,6 @@ static int __noreturn rcu_tasks_kthread(void *arg) > > > > > int rtst; > > > > > struct task_struct *t1; > > > > > > > > > > - schedule_timeout_interruptible(HZ); > > > > > rtst = READ_ONCE(rcu_task_stall_timeout); > > > > > needreport = rtst > 0 && > > > > > time_after(jiffies, lastreport + rtst); > > > > > @@ -768,6 +773,11 @@ static int __noreturn rcu_tasks_kthread(void *arg) > > > > > check_holdout_task(t, needreport, &firstreport); > > > > > cond_resched(); > > > > > } > > > > > + > > > > > + if (list_empty(&rcu_tasks_holdouts)) > > > > > + break; > > > > > + > > > > > + schedule_timeout_interruptible(HZ); > > > > > > Why is this a full second wait and not the HZ/10 like the others? > > > > The idea is to respond quickly on small idle systems and to reduce the > > number of possibly quite lengthy traversals of the task list otherwise. > > I actually considered exponential backoff, but decided to keep it simple, > > at least to start with. > > Ah, now it makes sense. Reading what you wrote, we can still do a > backoff and keep it simple. What about the patch below. It appears to > have the same performance improvement as Joel's Looks plausible to me! Joel, do you see any gotchas in Steve's patch? Thanx, Paul > -- Steve > > > > > > > > > Is there a better way to do this? Can this be converted into a for-loop? > > > > Alternatively, would it make sense to have a firsttime local variable > > > > initialized to true, to keep the schedule_timeout_interruptible() at > > > > the beginning of the loop, but skip it on the first pass through the loop? > > > > > > > > Don't get me wrong, what you have looks functionally correct, but > > > > duplicating the condition might cause problems later on, for example, > > > > should a bug fix be needed in the condition. > > > > > > > diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c > index 68fa19a5e7bd..c6df9fa916cf 100644 > --- a/kernel/rcu/update.c > +++ b/kernel/rcu/update.c > @@ -796,13 +796,22 @@ static int __noreturn rcu_tasks_kthread(void *arg) > * holdouts. When the list is empty, we are done. > */ > lastreport = jiffies; > - while (!list_empty(&rcu_tasks_holdouts)) { > + for (;;) { > bool firstreport; > bool needreport; > int rtst; > struct task_struct *t1; > + int fract = 15; > + > + /* Slowly back off waiting for holdouts */ > + schedule_timeout_interruptible(HZ/fract); > + > + if (list_empty(&rcu_tasks_holdouts)) > + break; > + > + if (fract > 1) > + fract--; > > - schedule_timeout_interruptible(HZ); > rtst = READ_ONCE(rcu_task_stall_timeout); > needreport = rtst > 0 && > time_after(jiffies, lastreport + rtst); >